The goal of the proposed research is to isolate and characterize cellular factors concerned with the mobility of nucleosomes in interphase and their close packing at mitosis. Factors that promote the sliding of nucleosomes are detected by the ligation of a nucleosome to a radioactive DNA molecule and subsequent protection of the radioactivity against nuclease digestion. Factors that promote nucleosome exchange will be assayed by the use of a simple mixture of nucleosomes and radioactive DNA. The packing of nucleosomes in metaphase chromosomes is under study by nuclease digestion and low angle X-ray diffraction. The role of histone modifications in this packing will be investigated. Factors that promote the packing and consequent chromosome condensation will be sought by in vitro complementation between nuclei and cytosol at different stages of the cell cycle. Friend erythroleukemia cells are the main source of material for this work.